Enter the account names of the agencies that you share with in the "New Address" box, click the "Add New" button, and it will be there whenever you need to share records. You need to enter account names just as they appear in the title field of the other agency's account (your title field appears right above your tabs in any of the main screens).
When it is time to share records, select the accounts you want to send to from your address book, and use the arrows to move them into the recipient list. Another great thing is that those same recipients will be in the list until you move them back out to the address book. So the next time you go to share records the same accounts will appear as recipients. If you share with the same group all of the time, you are already set.